1. Field of the Invention
This invention relates to antibacterial ceramic materials which are shown to be highly safe and manageable powders which can be admixed with resins, fibers, papers, ceramics and the like. These ceramics have been made to absorb and carry tightly a metal ion or metal salt, namely at least one metal salt of silver, zinc and copper, after which the same ceramic is heat fired at elevated temperatures, preferably 800.degree. C. or higher. The high temperatures cause the ceramic to undergo contraction and it is thereby treating the material in such a manner as to permanently lock-in the said metal ions or metal salts so that they will not desorb or elute into a contacting fluid.
2. Description of the Prior Art
It has long been known that metals selected from among silver, copper and zinc, as well as their metal ions and salts, exhibit a strong antibacterial property. Various processes have been proposed for utilizing this antibacterial property by providing a substrate, such as a textile or synthetic resin, with such a material by means of diffusion, adsorption or coating. However, since these metals exhibit poor dispersibility with respect to a substrate, it is difficult to disperse them in a substrate homogeneously, and therefore the article obtained tends to display a non-uniform quality and non-uniform antibacterial property. An article thus obtained by simply diffusing the metal salt in a substrate is such that the metal ion or metal salt elutes from the substrate in the presence of water. Consequently, the article suffers a decline in quality and antibacterial capability, and the eluted metal ion or metal salt can cause undesirable damage. In addition, for example, since salts such as silver salts readily change color when exposed to daylight, it is difficult to store them for long periods and articles to which these salts have been added may also undergo a change in color. Despite the fact that these metal ions or their salts are highly resistant to heat and possess a strong antibacterial property, the foregoing drawbacks preclude much use of these materials as antibacterial agents except in some very limited fields of application.
In recent years substances in which an ion exchanger is used as a carrier and the carrier undergoes an ion exchange with or absorption of the aforementioned metal ions or salts have been proposed as carriers that enable the antibacterial property of the metals, metal ions or metal salts to be utilized safely. For example, the specification of Japanese Patent Application Laid-Open (Kokai) No. 60-181002 discloses an antibacterial agent in which zeolite is made to undergo an ion exchange with these metal ions. By virtue of this ion exchange process, elution of the antibacterial metal ions into water is reduced, dispersibility in a substrate such as that of a textile, synthetic resin or the like is also improved, the drawbacks observed in the prior art when a metal, metal ion or salt is used as an antibacterial agent are therefore mitigated, and antibacterial metals can be utilized in comparative safety. However, in a process with an ion exchanger, such as zeolite, which is made to carry antibacterial metal ions by means of ion exchange, the amount of metal ions which can undergo the ion exchange is limited by the ion-exchange capacity of the carrier material and therefore it is necessary to use an ion exchanger having a large ion-exchange capacity in order to achieve a desired strong antibacterial property. A problem encountered however is that the types of ion exchangers that can be used are limited. In addition, the antibacterial metal ions carried on the ion exchanger as a result of the ion exchange cannot always be used with safety in any medium of choice because these metal ions may be released out of the carrier depending upon the particular type of carrier used. Further recently, in a zeolite antibacterial agent, obtained by an exchange of silver ions, discoloration has been shown to be reduced in comparison to a material in which a silver salt is simply adsorbed. However, since discoloration does occur with the passage of time, storage for long periods has proven difficult and articles to which this zeolite and metal salt has been added are likely to deteriorate owing to discoloration.
The specification of Japanese Patent Application Laid-Open No. 60-181002 discloses a method to heat treat the metal-ion exchanged into zeolite at a temperature of 340.degree.-580.degree. C., which is lower than that at which the zeolite would start to undergo thermal decomposition, with the purpose being to reduce the conversion of metal into metal oxide to limit the amount of gases evolved at the time of use. Consequently, when the metal-ion exchanged zeolite is used, depending on the conditions, use is facilitated since safety in terms of the composition has been enhanced somewhat in comparison with the metal-substituted zeolite that has not been heat treated. However there is no significant difference with regard to the release of metal ions and or discoloration with the passage of time. Moreover, if zeolite carrying silver is heat treated at a temperature of higher than 600.degree. C., which is greater than the temperatures of 340.degree.-580.degree. C. mentioned in Japanese Patent Application Laid-Open No. 60-181002, colors of gray to black are produced owing to the action of the silver, and this prevents its use in many needed applications.